1. Field of the Invention
The present invention relates to the composition, preparation, and use in elastomer compositions of latent mercaptosilane coupling agents containing siloxane bonds. These coupling agents represent an improvement over the prior art in that their use is accompanied by reduced volatile organic compound (VOC) emissions.
2. Description of Related Art
Latent mercaptosilane coupling agents known in the art contain hydrolyzable groups that are converted to volatile byproducts when the coupling agents react with the fillers used in the rubber compositions.
The majority of art in the use of sulfur-containing coupling agents in rubber involves silanes containing one or more of the following chemical bond types: S—H (mercapto), S—S (disulfide or polysulfide), or C═S (thiocarbonyl). Mercaptosilanes have offered superior coupling at substantially reduced loadings; however, their high chemical reactivity with organic polymers leads to unacceptably high viscosities during processing and premature curing (scorch). Their undesirability is aggravated by their odor. As a result, other, less reactive coupling agents have been found. Hence, a compromise must be found between coupling and the associated final properties, processability, and required loading levels, which invariably leads to the need to use substantially higher coupling agent loadings than would be required with mercaptosilanes, and often also to the need to deal with less than optimal processing conditions, both of which lead to higher costs.
Acylthioalkyl silanes, such as CH3C(═O)S(CH2)1-3Si(OR)3 and HOC(═O)CH2CH2C(═O)S(CH2)3Si(OC2H5)3 are disclosed in Voronkov, M. G. et al. in Inst. Org. Khim., Irkutsk, Russia and U.S. Pat. No. 3,922,436, respectively.
U.S. Pat. No. 3,957,718 discloses compositions containing silica, phenoplasts or aminoplasts, and silanes, such as xanthates, thioxanthates, and dithiocarbamates; however, it does not disclose or suggest the use of these silanes as latent mercaptosilane coupling agents, nor does it suggest or disclose the advantage of using them as a source of latent mercaptosilane.
U.S. Pat. Nos. 4,184,998 and 4,519,430 disclose the blocking of a mercaptosilane with an isocyanate to form a solid that is added to a tire composition, which mercaptan reacts into the tire during heating, which could happen at any time during processing since this is a thermal mechanism. The purpose of this silane is to avoid the sulfur smell of the mercaptosilane, not to improve the processing of the tire. Moreover, the isocyanate used has toxicity issues when used to make the silane and when released during rubber processing.
Australian Patent AU-A-10082/97 discloses the use in rubber of silanes of the structure represented byR1nX3-nSi—(Alk)m(Ar)p—S(C═O)—Rwhere R1 is phenyl or alkyl; X is halogen, alkoxy, cycloalkoxy, acyloxy, or OH; Alk is alkyl; Ar is aryl; R is alkyl, alkenyl, or aryl; n is 0 to 2; and m and p are each 0 or 1, but not both zero. This patent, however, stipulates that compositions of the structures of the above formula must be used in conjunction with functionalized siloxanes. In addition, the patent does not disclose or suggest the use of compounds of Formula (1P) as latent mercaptosilane coupling agents, nor does it disclose or suggest the use of these compounds in any way which would give rise to the advantages of using them as a source of latent mercaptosilane.
JP 63270751 A2 discloses the use of compounds represented by the general formula CH2═C(CH3)C(═O)S(CH2)1-6Si(OCH3)3 in tire tread compositions; but these compounds are not desirable because the unsaturation α,β to the carbonyl group of the thioester has the undesirable potential to polymerize during the compounding process or during storage.
There remains a need for effective latent coupling agents which exhibit the advantages of mercaptosilanes without exhibiting the disadvantages such as described herein.